The application field of metal oxide films has been diversified widely in recent years. They are being used, for example, as insulating films and orientation controlling films in liquid crystal displays, protective films for ceramics and metals, and insulating films for semiconductor devices. In particular, a liquid crystal display is fabricated by forming a patterned transparent conductive film on an insulating substrate such as glass substrate, forming a metal oxide film over the conductive film to construct an electrode substrate, arranging a pair of such electrode substrates, with such a metal oxide film formed thereon, side by side with a spacer interposed therebetween along the peripheries thereof so as to form a cell, and sealing a liquid crystal material within the cell.
As characteristics required for such metal oxide films, they are supposed to have high adhesion to insulating substrates and transparent conductive films and moreover, they should themselves be free of defects such as pinholes and should be uniform films. Since metal oxide films are used in various ways in recent years, the above-mentioned characteristics of metal oxide films must of course satisfy their individual application purposes. It is also a critical requirement that they have excellent mass producibility upon their formation. As processes known presently for the formation of a metal oxide film, may be mentioned vacuum deposition, CVD (chemical vapor deposition), sputtering, etc. However, these processes are all insufficient in mass producibility and can hardly be said to be suitable processes. For example, vacuum deposition employs a vacuum apparatus. In particular, when one wants to form a metal oxide film on a large substrate, the film thickness varies too much to obtain a uniform film. In addition, a large and expensive vacuum apparatus is indispensable so that the production cost increases. Such large metal oxide films lack mass producibility. CVD requires to heat a substrate upon formation of a metal oxide film thereon and is difficult to form a uniform film. An expensive apparatus is required for the practice of CVD. CVD is insufficient in mass producibility. On the other hand, sputtering has substantially the same drawbacks as vacuum deposition.
Coating processes have been proposed as processes which have improved the problems or drawbacks of the film-forming processes described above. In such coating processes, a coating solution for use in the formation of a metal oxide film is applied on a substrate and is then heated to form a metal oxide film. Coating processes have such advantages that a film can be formed easily irrespective of the size of a substrate and owing to simple procedures required for the formation of a film, the mass producibility is extremely high.
As a conventional coating solution known to be suitable for use in the formation of a metal oxide film by such a coating process, there is a coating solution prepared by adding polyethylene glycol or nitrocellulose to a solution composed of a metal complex of a .beta.-diketone and as a solvent, a .beta.-diketone-containing organic or inorganic solvent, e.g., a low-viscosity alcohol such as methanol, ethanol or propanol, a ketone such as acetone, or an ester (Japanese Patent Laid-Open No. 149920/1980). This coating solution permits easy formation of films on various substrates. However, films formed of this coating solution have high porosity. Decomposition intermediates and carbonic residues of polyethylene glycol or nitrocellulose which is contained in the coating solution may thus remain in pores even after a heat treatment applied subsequent to the coating of the solution, thereby preventing the formation of a uniform metal oxide film. Furthermore, the coating solution has poor storage stability so that a deposit occurs in the coating solution when it is left over for a long period of time.
It has heretofore been difficult to provide a coating solution which may be applied to any one of coating processes such as dip coating, spin coating, roll coating and transfer coating. A metal oxide film formed by a coating process which requires a coating solution of a relatively high viscosity is inferior in uniformity and/or poor in adherence to an insulating substrate or transparent conductive film. In order to obtain a thick metal oxide film, it is effective to apply a high-viscosity coating solution by a coating process such as roll coating or transfer printing. It is generally indispensable to incorporate a thickening agent such as organic polymer in order to prepare a high-viscosity coating solution. A coating solution with a thickening agent incorporated therein however involves such problems that the coating solution has poor storage stability and undergoes variations upon standing, thereby being gelled or developing a deposit and moreover, a metal oxide film to be formed will have low adhesion to the surface of its corresponding substrate, thereby causing peeling-off or clouding.